The coffer is probably inside the mason's workshop's inventory still. Use the building i(t)ems cursor to see that inventory, +-*/ to scroll to the coffer, then ENTER to bring up the item's screen. Alternatively, use the artifacts screen (the hotkey is l IIRC) to see it.
You have to (r)equest the trader to come to the depot, then have the trader come to the depot, before you can trade. Once the trader is there, you can (t)rade to bring up the trade screen, then select all of the items you want from the caravan and all of the items you want to trade them, then (t) again to trade them. Do NOT (o)ffer items! That is essentially you giving the items you have selected on your side to the caravan as a gift and you'll get nothing in return for them.
You probably neglected to actually trade with the dwarven caravan if you didn't do all that stuff above. The dwarven caravan comes with a trade liaison, who makes a trade agreement with you, which allows you to tell the next caravan what to bring next year.
First off, I should say that covering your mines with plain-stone flooring is mostly pointless. It doesn't actually add value if you're putting constructed stone floors over natural stone floors of the same material and prevents you from engraving it. I'd recommend against it; if you want to get the more attractive floor tile graphic, get an engraver, which is a dwarf with the Stone Detailing labor, active, then (d)esignate the floors for (s)moothing.
Second, your dwarves aren't cancelling the orders, but rather they're suspending them. There's a difference. The reason why they're doing this is because they try to build the floors out of a specific stone, which isn't the one that is on that particular tile. The stone that is on that particular tile is instead reserved for use for another tile, probably one further south and to the right, and your dwarves will absolutely refuse to move a stone that is reserved for use since if they did they'll lose track of it.
This information is probably too advanced for your understanding, but part of the reason why this happens is because when you set a large area of constructions to be built at once, it doesn't actually designate them all at once. It designates the one at the bottom-right first, and you choose the material for that one at that time, then it designates the next one up. When your dwarves go to build it, though, they go for the last one designated for construction (the top-left one) first, which might well have the material for the earlier-designated constructions underneath it, so they suspend the construction since they can't move the thing. If you don't understand that, don't worry overmuch, but it's an explanation for why that occurs.
Also, did you notice that you said that you had other questions, but then proceeded to not ask any more questions?